so far the conversion from an `Error` to a `TestAssertionFailure` meant
that all information about the source errors were lost. often, the
`Display` implementation for `Error` implementations does not include
the source information since that should instead be retrieved via
`source`. while this has not yet been made into an official API
guideline (see [rust-lang/api-guidelines#210]) this is nevertheless
being followed.

various crates like `anyhow` or `eyere` take care of pretty-printing the
error chain on failure, however this does not work with `googletest`
since `googletest::Result` has `TestAssertionFailure` as the error type
which swallows any `Error` and only keeps the message.

to resolve this a simple error chain implementation is added to the
`From` implementation which pretty-prints the error chain.

example:

```
Error: test3

Caused by:
1: test2
2: test1
```

fixes google#657

[rust-lang/api-guidelines#210]: rust-lang/api-guidelines#210